1. Field of the Invention
Spectrophotometers are used to measure color of an object by measuring the intensity of light emitted from the object, at a plurality of wavelengths in the visible spectrum which is commonly regarded as falling in the range of 400 nanometers to 700 nanometers. The measured light may be reflected from an object sample or transmitted through it or generated by it, as in the case of a color cathode ray tube.
2. Background Art
The measurement of color has become important in many industries and applications. For example, the color of paint on an automobile on the assembly line is tested to assure consistency. Color measurements are regularly made in color printed materials to assure consistency as well. With the increasing use of color in the desk-top publishing field, there is a need for an inexpensive compact, easy to use spectrophotometer for use in that industry to identify colors and to test for color consistency. Furthermore, color matching has become an important field for the use of spectrophotometers. For example, when damaged areas of an automobile are repaired and repainted, it is important to obtain a nearly perfect match of the new paint with the old. Similarly, in the sale of paints or colored materials for home and other building decorating purposes, it is important to be able to find a color in order to match a specific desired color. Thus, there is a need for compact, easy to use and relatively inexpensive color measuring devices for color identification and matching purposes.
The typical prior art spectrophotometer includes an integrating sphere or a single light source disposed at an angle of 45.degree. to the object sample, a device for selecting various wavelengths from light reflected from or transmitted through the sample, and one or more sensors for sensing light intensity at each of the wavelengths. A signal processing device receives the intensity information and derives spectral data representative of the color of the light received from the object sample being measured. One prior art arrangement for determining intensity at each of the plurality of wavelengths, in common use in present-day Spectrophotometers, is described in U.S. Pat. No. 4,968,143 issued on Nov. 6, 1990. That arrangement uses a diffraction grating by which light is resolved into a diffraction beam covering a wavelength span from 400 to 700 nanometers. The diffraction beam is sampled by a plurality of photocells which are arranged to receive light at various wavelengths at 10 0r 20 nanometer intervals or the like.
Another arrangement for generating light output at the different wavelengths, which seems to have fallen in disfavor in modern spectrophotometers, is described in U.S. Pat. No. 4,093,991 issued on Jun. 6, 1978. This arrangement comprises a rotatable, variable interference filter wedge monochrometer which is rotated at a constant speed by a synchronous motor. The system response at any angular position of the rotatable variable wedge monochrometer is proportional to the light emitted by the specimen at particular wavelengths of light passed through the wedge at the angular position in question. As the monochrometer is rotated, various monochromatic beams are projected onto an electro-optical detector. The wavelength passed by each angular position of the wedge is identified by a shaft decoder, which transmits pulses relating to the rate of rotation of the monochrometer wedge. A signal processor derives information regarding angular position of the wedge from the pulses to identify the wavelength of the light passed by the wedge.
A problem with the prior art devices is the cost and size of the apparatus for providing signals indicative of intensity at each of the various wavelengths of interest along the spectrum. This is true for the prior art arrangement using the diffraction grating and a multiplicity of detectors as well as for arrangements using the variable interference filter wedge-type device for providing monochromatic light beams at different wavelengths.
Integrating spheres are used in spectrophotometers and are known to provide an average reading of color over an exposed surface area. The integrating sphere is preferably used with reflective surfaces since the specular component of light reflected from a glossy surface may be insulated and subtracted from the color readings in order to obtain a more accurate indication of the color of the object to be measured. One such prior art integrating sphere is disclosed in U.S. Pat. No. 5,369,481 issued on Nov. 29, 1994. One problem with prior art integrating spheres is that light is conducted from an aperture by means of a bundle of optical fibers terminating on a plurality of filter elements in which each filter element is designed to pass only light of a selected wavelength in the visible spectrum. Transmitting light via a bundle of optical fibers has a disadvantage in that the light is typically not evenly distributed over the bundle of fibers resulting in an uneven distribution of light projected on the various filters of different wavelengths. Another disadvantage of light measurements by means of an integrating sphere is that it is often necessary to obtain a reading of the source light incident of the object sample in order to compensate for variations and diffuse light striking the sample and changing the readings obtained from the sample.